Airblast fuel injectors have been proposed for use in a staged gas turbine engine fuel systems. Airblast fuel injectors are described in the Helmrich U.S. Pat. No. 3,864,186 and others and are designed to achieve atomization of a film of liquid fuel formed on a fuel discharge surface or lip by directing high velocity airflow from the engine compressor at the fuel film as it leaves the orifice surface.
Airblast fuel injectors have been proposed for use in a staged gas turbine engine fuel system. However, use of airblast fuel injectors in a staged fuel system may present so-called coking problems when the airblast fuel injectors comprise the secondary (or main) injectors of the system that are rendered non-operative during some engine regimes (e.g. low engine power regimes such as low speed idle operations) and operative during other engine regimes (e.g. high speed operations). In particular, when the secondary fuel injectors are rendered non-operative during low power operation, fuel remains in the fuel discharge passages of the injectors and can be heated by the elevated temperature of the combustor environment to the extent that the fuel in the fuel discharge passages is carbonized or coked over time, building up carbonaceous deposits therein. These deposits can adversely affect the performance of the airblast fuel injectors in service from the standpoint that injector fuel flow rate and fuel atomization become unsatisfactory. Similar deposits may build-up in the fuel passages of other air atomizing fuel injectors, such as hybrid pressurizing/airblast injectors, that employ high velocity compressor airflow to atomize fuel discharged from a fuel discharge passage/lip.
U.S. Pat. No. 5,277,023 of common assignee herewith discloses a self-sustaining, fuel purging fuel injection system that employs a purging pressure differential established on fuel residing in interconnected fuel discharge passages of the injectors effective to purge fuel from the fuel discharge passages when the injectors become non-operative. The purging pressure differential is established at the fuel discharge lips of the injectors by controlled geometrical/air flow relationships between air/fuel lips of the injectors, rather than by inherent pressure differentials relative to the engine combustor.
It is an object of the present invention to provide a fuel injection system for a gas turbine engine wherein the system is provided with secondary or main fuel staging and purging capabilities wherein a pressure differential is used for purging and wherein purging is controlled by a purge valve referenced to pilot fuel inlet pressure.
It is an object of the present invention to provide a fuel injection system for a gas turbine engine wherein the system is provided with secondary or main fuel staging and purging capabilities wherein a pressure differential is used for purging and wherein purging is controlled by a purge valve in turn controlled by a three-way servo-controlled valve.
It is another object of the invention to provide a fuel injector having a shuttle valve for providing a fuel injection mode or fuel purging mode.